Ketamine is a non-selective NMDA receptor antagonist that has been approved by FDA for induction and maintenance of the general anesthesia. It has also been shown effective in treating other conditions, for example, to alleviate different kinds of pain (Correll, 2003), depression (Zarate, 2012), acute brain injury and stroke (Heftle, 2012), epilepsy (Synowiec, 2013), alcohol dependence, Alzheimer's disease, asthma and other disorders.
The oral efficacy of ketamine for treatment of pain has been confirmed by multiple investigators and recently reviewed (Blonk, 2010). In most cases, ketamine was used as an oral solution prepared from the commercially available injectable formulation (1 or 10% ketamine in water), often times mixed with fruit juice or syrup for taste masking. Solid dose forms of ketamine have also been reported in several examples. In particular, Yanagihara et al. (Yanagihara 1999, 2003) reported preparation of oral tablets of ketamine by dry and wet granulation with pharmacokinetics in humans similar to the orally administered syrup formulation. Furthermore, oral and sublingual formulations of ketamine as gelatin-based lozenges having a total weight of 1 g and ketamine load of 25 mg have also been prepared by Chong (Chong, 2009).
When administered orally, ketamine is a subject to the first-pass liver metabolism via N-demethylation and conversion to the active metabolite Norketamine. The elimination half-life of ketamine has been estimated at 2-3 hours, and 4 hours for norketamine. Consequently, the therapeutic window of orally administered ketamine is relatively short, and prompts an oral administration of multiple daily doses of the drug, e.g., 3-5 times a day, to achieve desirable therapeutic effect.
Moreover, solid dose forms of ketamine have been consistently limited by their inability to provide therapeutically effective doses, even in the short-term, without neurologically toxic spikes in ketamine concentration. In fact, exceeding an optimal efficacy plasma concentration of the drug (10-300 ng/ml) leads to more pronounced side effects, such as sedation, hallucination, dizziness, and/or nausea, which can not only have immediate repercussions, but also effect treatment compliance.
In order to achieve the optimal therapeutic index, the most successful route of administration for maintaining the stable levels of the drug in the system over longer periods of time appears to be by infusion (Correll, 2004). Such administration affords direct titration control of the manner of the administration, and enables eliminating the presence of neurological side effects, e.g., resulting from psychotomimetic toxic plasma concentration spikes of ketamine. However, the process of infusion presents significant challenges in patient management, as well as the cost of the procedure, being difficult to administer outside of the Intensive Care Units (ICU).
As such, there remains a need for efficient, more convenient, and controllable ketamine formulations that mimic the results of ketamine infusion and afford no neurologically toxic (e.g., psychotomimetic toxic) plasma concentrations, and which address the identified gap in ketamine treatment of conditions such as pain, depression, traumatic brain injury, stroke, epilepsy, alcohol dependence, or Alzheimer disease.